The present invention relates to an aging method suitable for a cathode-ray tube (CRT), especially, a large-area color picture tube or CRT, so that cold emission from an accelerating electrode upon actual use of the CRT can be reduced.
In recent years, there is seen an increasing tendency of color CRT's to large-sized ones and the application of relatively high voltages to an anode and a focusing electrode (usually, a G.sub.3 electrode) is required in order to suppress the lowering of the luminance of pictures resulting from an increase of the size of a screen of the CRT. As a result, even little contamination of electrodes is liable to cause cold emission therefrom so that the quality of an image or a picture displayed on the CRT screen may be deteriorated. Namely, even under a condition of operation in which the emission of a thermoelectron beam is cut off by a G.sub.1 electrode, electrons produced by the cold emission may excite phosphor substances on a phosphor screen of the CRT into fluorescence, thereby lowering the contrast of an image displayed. Also, the cold emission is one factor of a discharge in the tube and a strong discharge undesirably brings about a failure of a CRT driver circuit.
Usually, the cold emission generates concentratedly from a location at which the strength of an electric field is high. Therefore, a main source of the cold emission is a focusing electrode system to which a high voltage is applied. However, as the requirements for improvement on the quality of the CRT tube becomes severe, there has been increased the need of having another look at cold emission from a G.sub.2 electrode to which a low voltage is applied, though it was a common knowledge that the cold emission from the G.sub.2 electrode is rather low.
One of conventional methods of reducing the cold emission from the G.sub.2 electrode has been disclosed by JP-A-57-67261 laid open on Apr. 23, 1982, entitled "Method of Manufacturing Cathode-Ray Tube". In the disclosed method, the G.sub.2 electrode is cleaned by irradiating the vicinity of an opening of the G.sub.2 electrode with an electron beam during a process step for manufacture of the CRT. In this case, constant D.C. voltages are applied to various electrodes. Therefore, respective optimum voltage conditions must be established for different types of CRT's. The abovementioned cleaning process for the G.sub.2 electrode is one of kind of aging. As is well known, "aging" is a general term for a process for stabilization of the electron emitting characteristics of a cathode, a process for cleaning of the surface of each electrode, and so on which are carried out in a process step for manufacture of an electron tube in order that the electron tube can maintain a satisfactory operation over a long period of time in its practical use. The JP-A-57-67261 does not refer to undesirable influence of the bombardment of an accelerated electron beam on an opening of the focusing electrode system.
JP-A-57-38538 laid open on Mar. 3, 1982, entitled "Method of Aging Color CRT" has disclosed an aging method in which methane in gases emitted from electrodes in a CRT is ionized by the action of an electron beam during a process called "raster aging" in order to provide an ionized version of the methane liable to be absorbed by a barium getter, thereby improving the degree of vacuum in the tube and reducing a possibility that the surface of a cathode may be damaged by the impingement of gaseous ions thereagainst upon actual use of the CRT. In this aging method, too, D.C. voltages are applied to respective electrodes so that the electron beam impinges upon the electrodes uniformly at all times or only fixed locations thereof. Also, the JP-A-57-38538 refers to neither the cleaning of a G.sub.2 electrode nor the reduction of cold emission therefrom.